The invention relates to communication systems. More particularly, the invention relates to methods and apparatus for scheduling or assigning resources such as rate and power in a wireless communication system.
Several multiple access communication techniques are known in the art, such as time division multiple access (TDMA) and frequency division multiple access (FDMA). However, the spread spectrum modulation techniques of code division multiple access (CDMA) provide significant advantages over other multiple access modulation techniques. CDMA techniques in a communication system are disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention.
Since CDMA employs a wideband signal, it spreads the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. CDMA also provides space or path diversity through multiple signal paths that simultaneously link a mobile station or user with two or more cell-sites. Furthermore, CDMA can exploit the multipath environment by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention.
CDMA modulation techniques require that all transmitters be under precise power control to manage interference in the system. If the transmission power of signals transmitted by a base station to a user (the forward link) are too high, it can create problems such as interfering with other users. Most base stations have a fixed amount of power at which to transmit signals, and therefore can transmit to only a limited number of users. Alternatively, if the transmission power of signals transmitted by the base station is too low, then some users can receive multiple erroneous transmitted frames. Terrestrial channel fading and other known factors also affect the transmission power of signals transmitted by the base station. Thus, each base station needs to adjust the transmission power of the signals it transmits to its users. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention.
Under one CDMA standard, described in the Telecommunications Industry Association""s TIA/EIA/IS-95-A Mobile Stations-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, each base station transmits pilot, sync, paging and forward traffic channels to its users. Under this standard, power control signals or codes are also exchanged between each base station and the mobile stations to provide appropriate power control for the system.
Improvements to the above standard have included additional, higher data rates. These higher data rates help provide for data services beyond traditional voice services. Voice services typically tolerate higher error rates than data services (e.g., a maximum bit error rate (BER) of 10xe2x88x923), but require continuous bit stream transmissions with no delays. Most data, such as electronic mail, facsimile and general computer data, may use discontinuous, packetized data transmissions. Such data typically must be transmitted at bit rates higher than speech, but are insensitive to delay and require lower error rates. For example, facsimile, general computer data and email typically are transmitted at bit rates of 8-32 kbps, 0.1-1 Mbps, and 9.6-128 kbps, and maximum BER""s of 10xe2x88x924, 10xe2x88x929 and 10xe2x88x929, all respectively. Video requires even higher bit rates and lower error rates than voice, and, like voice, requires continuous bit stream transmissions. For example, low resolution video typically requires a bit rate of 64-128 kbps and a maximum BER of 10xe2x88x925.
To be efficient, a wireless communication system must not provide the same data rate, error rate and bit stream (power) for all services based on the most stringent requirements of any one service. Therefore, one prior technique employs dynamic control algorithms for admission or registration control, resource allocation and error recovery and at burst or packet levels for a given base station. See, e.g., A. Sampath, P. Kumar and J. Holtzman, xe2x80x9cPower Control and Resource Management for a Multimedia less than CDMA Wireless System,xe2x80x9d PIMRC, 1995. Such a system, however, may provide ad hoc or immediate service allocation, which is not efficient or optimized. Each new service request is allocated at that time by the base station. Additionally, while one base station may optimize itself for an immediate service allocation, such optimizations may well create interference for adjacent base stations. If one base station is optimizing itself, interference it receives from an adjacent base station, (which is itself optimizing) can cause two adjacent base stations to continually create interference for each other and thereby result in an unstable condition within the wireless communication system.
One solution to the possible problem of interference between base stations or cell sites during resource optimization, such as rate and power optimization, is to employ a central processor or selector that synchronously controls each cell. A centralized controller, however, requires complex computations for each cell, and the computational burden grows exponentially with each additional cell. Moreover, a centralized controller requires information to be transmitted between base stations, as well as to the centralized controller. Furthermore, such a centralized controller may require that all base stations perform interference measurement and rate assignment synchronously, thereby further increasing the complexity of such a centralized approach.
The inventors have developed a technique where each base station performs the rate assignment optimally but independently of the other base stations. Different base stations affect each other through other cell interference, and continuously modify their reverse link rate assignment based on the other-cell interference received and the requested rates from the mobile stations. Under the inventors"" technique, the base stations converge to a stable condition with uncoordinated optimizations (i.e., without a central processor).
Under one embodiment of the invention, a distributed reverse link rate assignment technique assigns reverse link rates optimally within each cell, while also maintaining interference to other cells at a minimum level. The optimization technique maximizes the total throughput in each cell subject to a set of constraints, such as the following constraints: mobile station""s maximum transmit power, mobile station""s requested rate, discrete set of possible rates, maximum rise-over-thermal interference at the base station, and a minimum required received energy per bit normalized for noise (Eb/N0).
Each base station assigns rates in such a way that minimizes other-cell interference by assigning higher rates to mobile stations closer to the center of the cell, and lower rates to mobile stations further from the center of the cell.
In a broad sense, one aspect of the invention embodies a communications system having at least first and second base stations exchanging communication signals with at least first and second user stations, respectively. A method under the communication system includes: (a) receiving transmission requests from the first and second user stations, respectively, and scheduling requests received from other user stations, wherein the first base station optimizes the scheduling independently of the scheduling of the second base station and minimizes interference with the second base station, and vice-versa, and (b) transmitting first and second assignment signals to the first and second user stations respectively, wherein the assignment signals specify at least one transmission criteria at which the user stations are to transmit data.